The quality and performance of a digital fiber optic transmitter is determined by the distance over which the transmitted digital signal can propagate without severe distortions. The bit error rate (BER) of the signal is measured at a receiver after propagation through dispersive fiber and the optical power required to obtain a certain BER, typically 10−12, called the sensitivity, is determined. The difference in sensitivity at the output of the transmitter with the sensitivity after propagation is called the dispersion penalty. This is typically characterized by the distance over which a dispersion penalty reaches a level of ˜1 dB. A standard 10 Gb/s optical digital transmitter, such as an externally modulated light source (e.g., a laser), can typically transmit up to a distance of ˜50 km in standard single mode fiber, at 1550 nm, before the dispersion penalty reaches the level of ˜1 dB, called the dispersion limit. The dispersion limit is determined by the fundamental assumption that the digital signal is transform-limited, i.e., the signal has no time-varying phase across its bits and has a bit period of 100 ps, or 1/(bit rate), for a 10 Gb/s system. Another measure of the quality of a transmitter is the absolute sensitivity after fiber propagation.
Three types of optical transmitters are presently in use in prior art fiber optic systems: (i) directly modulated laser (DML) transmitters, (ii) electroabsorption modulated laser (EML) transmitters, and (iii) externally modulated Mach Zhender (MZ) transmitters. For transmission in standard single mode fiber at 10 Gb/s, and 1550 nm, it has generally been believed that MZ transmitters and EML transmitters can have the longest reach, typically reaching 80 km. Using a special coding scheme, referred to as phase shaped duobinary coding, MZ transmitters can reach 200 km. On the other hand, directly modulated laser (DML) transmitters typically reach <5 km, because their inherent time-dependent chirp (i.e., frequency shifts) causes severe distortion of the signal after this distance.
Various systems have been developed which increase the reach of DML transmitters so as to make them practical for long-reach lightwave data transmission (e.g., >80 km at 10 Gb/s). By way of example but not limitation, long-reach DML transmitters capable of transmitting >80 km at 10 Gb/s in single mode fiber are disclosed in (i) U.S. patent application Ser. No. 10/289,944, filed Nov. 6, 2002 by Daniel Mahgerefteh et al. for POWER SOURCE FOR A DISPERSION COMPENSATION FIBER OPTIC SYSTEM, now U.S. Pat. No. 6,963,685; (ii) U.S. patent application Ser. No. 10/680,607, filed Oct. 6, 2003 by Daniel Mahgerefteh et al. for FLAT DISPERSION FREQUENCY DISCRIMINATOR (FDFD), now U.S. Pat. No. 7,054,538; and (iii) U.S. patent application Ser. No. 10/308,522, filed Dec. 3, 2002 by Daniel Mahgerefteh et al. for HIGH-SPEED TRANSMISSION SYSTEM COMPRISING A COUPLED MULTI-CAVITY OPTICAL DISCRIMINATOR; which three patent applications are hereby incorporated herein by reference. The transmitters associated with these novel long-reach DML systems are sometimes referred to as a chirp-managed laser (CML™) transmitter by Azna LLC/Finisar Corporation of Wilmington, Mass.
In these Azna/Finisar chirp-managed laser transmitters, a frequency modulated (FM) source is followed by an optical spectrum reshaper (OSR) which uses the frequency modulation to increase the amplitude modulated signal and partially compensate for dispersion in the transmission fiber. In one embodiment, the frequency modulated source may comprise a directly modulated laser (DML). The optical spectrum reshaper (OSR), sometimes referred to as a frequency discriminator, can be formed by an appropriate optical element that has a wavelength-dependent transmission function (e.g., a filter). The OSR can be adapted to convert frequency modulation to amplitude modulation.
In the novel system of the present invention, which is based on the aforementioned chirp-managed laser (CML™) transmitters of Azna LLC/Finisar Corporation, the chirp properties of the frequency modulated source (e.g., laser) are separately configured, and then further reshaped, by configuring the OSR (e.g., filter) to further extend the reach of the chirp-managed laser transmitter to over 250 km on standard single mode fiber at 10 Gb/s and 1550 nm. The novel system of the present invention preferably also combines, among other things, selected features of chirp-managed laser transmitter systems described in (i) U.S. patent application Ser. No. 11/068,032, filed Feb. 28, 2005 by Daniel Mahgerefteh et al. for entitled OPTICAL SYSTEM COMPRISING AN FM SOURCE AND A SPECTRAL RESHAPING ELEMENT, now U.S. Pat. No. 7,555,225 and (ii) U.S. patent application Ser. No. 11/084,630, filed Mar. 18, 2005 by Daniel Mahgerefteh et al. for FLAT-TOPPED CHIRP INDUCED BY OPTICAL FILTER EDGE, now U.S. Pat. No. 7,406,266, which two patents are hereby incorporated herein by reference.